What Makes a Fire Smoke?
The smoke that rises from a chimney or a bonfire is made chiefly of gases that are produced when the fuel burns. Most of the gases in wood smoke are water vapor and carbon dioxide. These invisible gases rise, and drift away with the air.
There are four things that you find in any piece of wood:
Water – Freshly cut wood contains a lot of water (sometimes more than half of its weight is water). Seasoned wood (wood that has been allowed to sit for a year or two) or kiln-dried wood contains a lot less water, but it still contains some.
Volatile organic compounds – When the tree was alive, it contained sap and a wide variety of volatile hydrocarbons in its cells. A compound is “volatile” if it evaporates when heated. These compounds are all combustible (gasoline and alcohol are, after all, hydrocarbons — the volatile hydrocarbons in wood burn the same way).
Carbon & Ash – Ash is the non-burnable minerals in the tree’s cells, like calcium, potassium and magnesium.
When you put the fresh piece of wood on a hot fire, the smoke you see is those volatile hydrocarbons evaporating from the wood. They start vaporizing at a temperature of about 300 degrees F (149 degrees Celsius).
Usually smoke has in it, too, tiny specks of carbon. They are partially burned bits of the fuel that is burning. The carbon, called soot, colors the smoke gray or almost black.
Some of the soot sticks to the walls of the chimney, blackening it. In many cities, the soot from burning fuels sometimes mixes with fog, forming “smog.”